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post #1 of 19 Old 06-12-2016, 12:39 PM Thread Starter
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Tire width vs. Wheel width

Hello all,

My 99 GT is my track car and I run HPDE events with a buddy of mine who has a Miata. Tire width and wheel width have come up in conversation and I wanted to put it out there to other Mustang guys.

He just bought some 15x8 wheels and will be running 205 tires on them (don't know profile). I said I was shocked, stock 17x8 Mustangs came with 245s on them, etc.

I have 275/40/17x9s on my '99 and he is convinced they're too big for a 9" wide rim. Upon further investigation and research it seems he's probably right - most road racers actually want the tire slightly stretched to fit the rim. An explanation from someone on corner-carvers said that the reason for this is that the sidewall will deflect under the cornering loads of the car and start to roll under, putting the sidewall in tension, which actually turns the car. When you have fat rubber on skinny rims, the beginning of sidewall deflection actually puts the sidewall in compression before rolling under more and putting the sidewall in tension, which makes for squirrely turn-in, etc. Stretching the tires slightly yields no compression-to-tension transition, its all tension all the time. In the same explanation he stated that Z06s run 275s on an 11" rim, Vipers run their steam roller 335s on 13" rims, etc.

'00 Cobra Rs run 265s on a 9.5" rim, so that's 10mm narrower on a .5" wider rim than I have. If I'm not mistaken, '95 Cobra Rs run 255/40s on a 9 inch rim.

So it appears the goal would be to look at the rim width range on the tire, as specified by the manufacturer, and put that tire on the widest rim specified.

It just so happens most 245/45/17s out there max out on a 9" rim. Smaller tires will also be lighter and cheaper, which is great for amateur level guys paying out of pocket for everything.

So, where does everyone stand on this? I am trying to get away from the mustang bro-science of jamming the fattest rubber possible on their cars for the looks. But I have also been pleased so far with my 275/40/17x9s, but I'm still a noobie with only 4 HPDE events under his belt.


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post #2 of 19 Old 06-12-2016, 01:53 PM
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I'm interested to see what others have to say based on their real-world observations.

IMO, another consideration is tire size vs. vehicle size/weight. A 245/45 might be "better" than a 275/40 on a 17x9" wheel, but 245s aren't much tire on a car that's probably in the 3400-3500# ballpark (with driver).

When I first started running my CTS-V, I ran 245/40-18s on 18x8.5" wheels. I then bought some 18x9" wheels and have been running 275/35-18s on those. Lap times are significantly better (multiple seconds per lap). Again, 245s might be the optimum choice for the 18x9" wheels, but I think the smaller 245s were just overwhelmed by the ~4000# car.


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post #3 of 19 Old 06-12-2016, 05:16 PM
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I have always ran the standard 275-40-17 on my 9" rims. I've never been on track with anything smaller and due to the weight of my fox (3400lbs) I feel that the biggest tire I can fit on my race rims, without roll over, serves me the best.

Judging by the side walls of my tires (street tires & R Comps) I see no evidence of significant roll over.

I would say a 10" rim on the same tires might possibly make turn in a little crisper, as well as make the same tires foot print a tad wider, but I would be surprised if it would actually impact lap times by anything significant.

I could be wrong though....

If I were to get a 17x10 with the right back spacing, more than likely lowering my lap times would probably consist of something bigger than a 275 on my now wider 10" rim.


Interestingly enough I just went over to tirerack.com and looked at some specs on 275-40-17 Street tires, R-Comps, and full slicks. Every single one and every single manufacturer recommended a rim width of 9"-11". So, it appears we are within the manufacturers recommended rim width for the tires we run. We might be at the bottom of acceptable width but we are in there and judging by the side walls of my tires, it appears that a 9" rim with that size tire is adequate enough to get the job done correctly.

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post #4 of 19 Old 06-12-2016, 07:09 PM Thread Starter
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Quote:
Originally Posted by Patrick Olsen View Post
I'm interested to see what others have to say based on their real-world observations.

IMO, another consideration is tire size vs. vehicle size/weight. A 245/45 might be "better" than a 275/40 on a 17x9" wheel, but 245s aren't much tire on a car that's probably in the 3400-3500# ballpark (with driver).

When I first started running my CTS-V, I ran 245/40-18s on 18x8.5" wheels. I then bought some 18x9" wheels and have been running 275/35-18s on those. Lap times are significantly better (multiple seconds per lap). Again, 245s might be the optimum choice for the 18x9" wheels, but I think the smaller 245s were just overwhelmed by the ~4000# car.
Yes, I spoke to that point with my Miata buddy and that's why I came here: to get feedback from (a) guys who track heavier cars, and (b) Mustangs, specifically.

Quote:
Originally Posted by LT1HAHA View Post
I have always ran the standard 275-40-17 on my 9" rims. I've never been on track with anything smaller and due to the weight of my fox (3400lbs) I feel that the biggest tire I can fit on my race rims, without roll over, serves me the best.

Judging by the side walls of my tires (street tires & R Comps) I see no evidence of significant roll over.

I would say a 10" rim on the same tires might possibly make turn in a little crisper, as well as make the same tires foot print a tad wider, but I would be surprised if it would actually impact lap times by anything significant.

I could be wrong though....

If I were to get a 17x10 with the right back spacing, more than likely lowering my lap times would probably consist of something bigger than a 275 on my now wider 10" rim.


Interestingly enough I just went over to tirerack.com and looked at some specs on 275-40-17 Street tires, R-Comps, and full slicks. Every single one and every single manufacturer recommended a rim width of 9"-11". So, it appears we are within the manufacturers recommended rim width for the tires we run. We might be at the bottom of acceptable width but we are in there and judging by the side walls of my tires, it appears that a 9" rim with that size tire is adequate enough to get the job done correctly.
I will probably end up getting some datalogging for lap times and whatnot either this year or next. I might go down to 245/45 or maybe 255/45 or something and compare. I believe there's something to this whole sidewall tension theory but its tough when you're the amateur guy paying for everything out of pocket because I can't buy 6 different tires in three different tire sizes to determine whats best lol.

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post #5 of 19 Old 06-13-2016, 07:52 AM
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I have run both 245s and 275s on 17x9s in the same tire and found that the 275s had more grip but the 245s felt "crisper" on turn in and directional changes. I now am using 275s on a 17x9 1/2.

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post #6 of 19 Old 06-13-2016, 08:47 AM
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i have always been a firm believer of 245's for a 9" wheel, and 275s for a 10/10.5" wheel

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post #7 of 19 Old 06-13-2016, 10:10 AM
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More tire, more tire, more tire. Miatas are so light and under-powered that they can get away with the small stretched tires and benefit from them. They are the exception, not the rule, imo. I would never recommend running any thing less than a 255 on a 9" rim. 245s are puny and belong on 8" rims (and even then, I'd probably run a 255).

Yes, a 275 will work better on a 10" rim than a 9" rim. However, imo, it would be slower than having 285s or 305s on the same 10" rim.
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post #8 of 19 Old 06-13-2016, 11:05 AM
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read away...... How to PROPERLY select and size TIRES for PERFORMANCE > MotoIQ - Automotive Tech, Project Cars, Performance & Motorsports

i run 275's on a 10.5" wheel which is slighty over stretched. but it also depends on the tire you are running. i have 305's on the same wheels and they are barely stretched
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post #9 of 19 Old 06-13-2016, 11:16 AM
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From what I understand another consideration is a wider rim for a given tire width also keeps the tread flat in relation to the road. When cornering hard with a tire that is too wide for your rim the tire will buckle and end up cup shaped with only the edges having good contact. A little extra tire pressure can help reduce this, but a proper width rim will be the better option.

I currently run a 245 on an 8" rim, unless I run my pressures about 15psi above street use I get a saw tooth pattern across the tread width. Next up for me are some 9" rims and even stickier 245s.

I prefer the wide look with the tire square on the rim, but is is squishy on the track. One other car in my group was a new GT350 his stock tires are stretched on the rim.

Just my $.02
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post #10 of 19 Old 06-13-2016, 11:22 AM
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My track setup, im running 18x10 with 275s and its dead nuts perfect.


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post #11 of 19 Old 06-13-2016, 02:58 PM
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Quote:
Originally Posted by 02 roush stage 3 View Post
read away...... How to PROPERLY select and size TIRES for PERFORMANCE > MotoIQ - Automotive Tech, Project Cars, Performance & Motorsports

i run 275's on a 10.5" wheel which is slighty over stretched. but it also depends on the tire you are running. i have 305's on the same wheels and they are barely stretched
This is a good guide. The main point to consider is a 275 in one brand could be an inch different from a 275 of another brand.
I usually check on tirerack to get the measured width and I try to keep it within a half inch of my wheel width.
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post #12 of 19 Old 06-13-2016, 04:07 PM
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Those running 10" rims on a fox, what is your back spacing/ offset?

What fender modifications were required to make them fit?

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post #13 of 19 Old 06-14-2016, 02:54 PM
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Quote:
Originally Posted by j rick kirby View Post
I have run both 245s and 275s on 17x9s in the same tire and found that the 275s had more grip but the 245s felt "crisper" on turn in and directional changes. I now am using 275s on a 17x9 1/2.
I have done this experiment as well and got the same results, and made the same decision. Much of what the OP said is right, but with the weight-to-power and weight-to-contact patch ratios most fendered cars are running, wider is going to be better in the long run, even if you give up transitional stability, turn-in response, etc.

One of the additional issues with narrow tires is that they run at higher temps than big tires due to higher contact pressure and lower mass/area/etc. (similar to brake rotors). So even if the smaller tire *might* have been faster on the first lap due to it's lower rotating inertia, stiffer sidewall, and crisper transitions, the big tire will be faster by the end because they reach lower temps and so maintain grip longer through a session.

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post #14 of 19 Old 06-14-2016, 06:41 PM
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I did back to back Maxxis RC-1 255's then 275's on 17x9 rims on the same day, an hour apart. I could turn in better and had more cornering speed than the 275. I have 280hp/280tq in a 3450lbs 98 Cobra. The 275 felt like I could get back to the throttle a little sooner coming out of the corner, but time was already lost in each corner. 1:04.1 with the 255 and 1:04.7 in the 275 flavor(best laps). The 255 weras out maybe a day or two faster, but I go 10.1 tenths at all times as I run race group and time trials. My race group class is limited to a 255 wide tire.

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post #15 of 19 Old 06-14-2016, 10:52 PM
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Quote:
Originally Posted by LT1HAHA View Post
Those running 10" rims on a fox, what is your back spacing/ offset?

What fender modifications were required to make them fit?
I've got posts and pictures floating around but here's the short version:
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post #16 of 19 Old 08-17-2016, 07:06 PM
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5.0Torx - With only four events under your belt and no autocross experience that I know of, you're going to be happy because (as near as I can tell) you haven't done enough driving at that hard of a level. Hard street driving, or what you may think was hard street driving, doesn't count for much here. No flame intended.


For all of the cars I ever bought aftermarket wheels for, the wheels were always picked in a width out toward max recommended for the intended tire size, and that's a history that reaches back 40+ years. Mainly for the improvements in turn-in and steering precision. Tire sizes themselves were picked with car weight in mind first.

Rather recently I picked up an Aim Solo datalogger, so I can put some numbers to go with evaluations of subjective feel. On my '08 GT, with (at the time) Koni yellows and firmer sta-bars and a moderately aggressive alignment, I tested my street wheel/tire set (MPSS in 265/40-18 on 18x9.5 wheels) and compared it to my track wheel/tire set (MPSS 285/35-18 on 18x11's). Same sizes all around, no width staggering for me. On one of my unofficial "test loops", the track setup was easier to drive at and more composed at 0.91 lateral g than the street setup was at 0.84 lateral g. I can explain only part of the difference as coming from the roughly quarter inch difference in sidewall height.

The track setup has datalogged in excess of 1.3g on a pretty flat track (NJMP). Even accounting for body roll introducing datalogging error, that's still about 1.2g or so, on 300 treadwear tires. Granted, this was on tires that were fully up to track temperature, but it was repeatable as long as I did my part.

I'll stick my neck out a bit to suggest that fitting tires to max-recommended width wheels probably demands a smoother driving style. Otherwise, any extra overshoot in roll could pick up more of the inside tread region of the outboard front tire.

I really doubt that I'd ever choose to run 275/40's on anything narrower than 10". Hell, I'd have 10's for the 265/40's if I was starting from scratch (the 9.5's were bought with 255/45's specifically in mind, just like what the 2008 GT500 wore up front). Apparently Ford was being unusually generous back around 2008; in addition to the GT500 fitment the "regular" GT got 235/50-18's on 8.5" wides.


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post #17 of 19 Old 08-19-2016, 11:05 AM
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I'm a firm believer that the tread width (not section width) of your tire should be roughly the same as your wheel, usually within half an inch. A slightly smaller tire will feel more crisp but might give less overall grip on a heavier car like a mustang. You can also go too big and then you get sloppy sidewalls, bulge, and the edges of the tires usually won't touch the road (except when they're folding over from being too big lol)
My personal preference is 255 or 265 on a 9, 275 on a 9.5, 285 on a 10, 295 on a 10.5, 305 or 315 on an 11.
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post #18 of 19 Old 08-20-2016, 09:02 AM
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I agree with that to the extent that you shouldn't intentionally choose "down" a tire size if the result becomes a tire that isn't enough for the car's weight.

But I think since you normally spend more time in a transient state as you approach and depart from the peak than you do at that peak, that response and precision are probably more important than the last couple of hundredths in peak grip in most real-world driving situations. Here's a trace of lateral acceleration (red), longitudinal acceleration (), and speed (green) taken from a lap at a track day (NJMP Thunderbolt, from just before T1 up to a little past T3).





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post #19 of 19 Old 08-26-2016, 08:18 PM Thread Starter
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Quote:
Originally Posted by Norm Peterson View Post
5.0Torx - With only four events under your belt and no autocross experience that I know of, you're going to be happy because (as near as I can tell) you haven't done enough driving at that hard of a level. Hard street driving, or what you may think was hard street driving, doesn't count for much here. No flame intended.


For all of the cars I ever bought aftermarket wheels for, the wheels were always picked in a width out toward max recommended for the intended tire size, and that's a history that reaches back 40+ years. Mainly for the improvements in turn-in and steering precision. Tire sizes themselves were picked with car weight in mind first.

Rather recently I picked up an Aim Solo datalogger, so I can put some numbers to go with evaluations of subjective feel. On my '08 GT, with (at the time) Koni yellows and firmer sta-bars and a moderately aggressive alignment, I tested my street wheel/tire set (MPSS in 265/40-18 on 18x9.5 wheels) and compared it to my track wheel/tire set (MPSS 285/35-18 on 18x11's). Same sizes all around, no width staggering for me. On one of my unofficial "test loops", the track setup was easier to drive at and more composed at 0.91 lateral g than the street setup was at 0.84 lateral g. I can explain only part of the difference as coming from the roughly quarter inch difference in sidewall height.

The track setup has datalogged in excess of 1.3g on a pretty flat track (NJMP). Even accounting for body roll introducing datalogging error, that's still about 1.2g or so, on 300 treadwear tires. Granted, this was on tires that were fully up to track temperature, but it was repeatable as long as I did my part.

I'll stick my neck out a bit to suggest that fitting tires to max-recommended width wheels probably demands a smoother driving style. Otherwise, any extra overshoot in roll could pick up more of the inside tread region of the outboard front tire.

I really doubt that I'd ever choose to run 275/40's on anything narrower than 10". Hell, I'd have 10's for the 265/40's if I was starting from scratch (the 9.5's were bought with 255/45's specifically in mind, just like what the 2008 GT500 wore up front). Apparently Ford was being unusually generous back around 2008; in addition to the GT500 fitment the "regular" GT got 235/50-18's on 8.5" wides.


Norm
Norm, thanks for the reply. Good science is good observation, so its good to hear from someone that has put the time and effort into getting good data. I don't take any offense to your comment on my skill level. I know full well that a pro driver in a half-broken Miata with 2 cylinders unplugged could smoke my ass lol. I'm learning and getting better every time I go out and I'm having a blast doing it, that's enough for me at this time.

I mean no disrespect either when I say I am curious as to where you comment on "hard street driving" came from, though. I never bought my 99 GT for that purpose. The previous owner of my car ran it in NASA TTC and I scooped it up when he moved out of state. My dd is my '94 5.0 GT which I'm done some little stuff to, nothing as major as what's done to the '99 (MM TA+PHB, Cobra brakes, other stuff), and I actually did do my very first motorsports thing ever, my one and only autox event, in my '94. After that I bought the '99 and have been doing track days ever since. I honestly dont really have any interest in doing any more autox. I know there's alot of overlap and also skills unique to autox that you might not get on a track, but, the dollar-to-drive-time ratio of track days absolutely creams autox, and you also get way more opportunity to learn or try new things from lap to lap, rather than having to wait another hour. Anyways, not quite sure where the street driving comment came from but I only posted this thread for road racing / track days driving.

After reading this thread I think my next tires will probably be 255/40s on my 17x9s because I dont care to drop $1000+ on all new bigger wheels and tires.

This past weekend I ran my car at Barber Motorsports Park, so, I'm up to 5 TDs now!

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